A display device displays an image according to display data given by an external system. However, in order to improve a displaying ability of the display device, a data conversion device for converting data may be provided between the external system and the display device.
An example of the improvement in the displaying ability, which is the purpose of such a data conversion process, is an improvement in response speed of a liquid crystal display device. As for most of liquid crystal display devices, the response speed is in a range of 10 ms to 30 ms in a case where a display condition changes from black to white or from white to black, and it is in a range of about 100 ms to 200 ms in a case where a display condition slowly changes from a half tone to a half tone. A frame frequency used in normal display is 50 Hz through 60 Hz, and therefore one cycle is 16.7 ms through 20.0 ms. Because the response speed is longer than the cycle of a single frame, transcription of display is not entirely carried out when a moving image such as a television is displayed, so that an after-image is generated.
An example of countermeasure against the problem is a data conversion process disclosed in Japanese Examined Patent Publication No. 25556/1988 (Tokukoushou 63-25556) (published date; May 25, 1988) and Publication of Patent No. 3167351 (Tokkyo 3167351) (published date; May 21, 2001). Such a data conversion process is referred to as overdrive mode driving or overshoot mode driving. Hereinafter, the data conversion process is referred to merely as overdrive mode driving.
Each of these processes uses a frame memory so as to perform a data conversion process between frames. This frame memory has a large hardware size. In order to minify the hardware size, Japanese Laid-Open Patent Publication No. 167555/2003 (Tokukai 2003-167555) (published date; Jun. 13, 2003) discloses a process for reducing the hardware size by compressing and coding data before writing it in a frame memory.
However, in case of simply compressing frame data, this results in a serious decrease in display quality. As for normal image information, as described in Japanese Laid-Open Patent Publication 167555/2003, when RGB data each of which is 8 bits are made into data as R of 5 bits, G of 6 bits, and B of 5 bits, only simple quantization error occurs. But in a case of the data conversion process between frames, when RGB data each of which is 8 bits are compressed into data as R of 5 bits, G of 6 bits, and B of 5 bits, then changed into RGB data each of which is 8 bits again and made into data of a previous frame, and the data conversion process between frames is performed using the data as a reference, a complicated quantization error may occur. This is because (1) as for the response speed of crystal liquid, response time becomes nonlinear due to combination of luminance at a beginning of a change and luminance at an end of the change, so that information of a previous frame becomes physically large; and (2) in a case of a simple quantization error, it merely seems to human eyes that color has become lighter, but in a case when sub pixels of red, green and blue which make up a pixel respectively change with multiple vectors, changes of the sub pixels are likely to be seen by human eyes, so that the error becomes complicated. Therefore, in a case of the overdrive mode driving, the correction amount must be increased in a specific combination of luminance at a beginning of a change and luminance at an end of the change. Thus, even when the RGB data are compressed into 6-bit R data, 8-bit G data, and 6-bit B data, let alone 5-bit R data, 6-bit G data, and 5-bit B data, a problem of being seen by human eyes, such as being seen with additional colors, occurs. This is a logical conclusion as follows. Namely, the reason this problem occurs is that the overdrive mode driving uses a method for improving the response speed by further changing, over an original change, a part whose response speed is lower than a change between black and white like a change from a half tone to a half tone. Thus, a shift of an original value is apt to be seen by human eyes, as explained above. In the case of Japanese Laid-Open Patent Publication 167555/2003, data is further thinned out from here, so that a worse effect is brought about.